We note that the polaronic transport is favored in materials with strong SOC that lack inversion symmetry [1]}. To get insight into the relevance of SOC, we further performed first-principles calculations using density function theory. We applied the WIEN2K [2]} implementation of the full potential linearized augmented plane wave method in the generalized gradient approximation using the PBEsol functional [3]} on stoichiometric IrSbSe [4]}. The SOC is treated in the second variation method. The basis size was determined by \(R_\mathrm {mt}K_\mathrm {max}\)  = 7 and the Brillouin zone was sampled with a regular \(15\times 15 \times 15\)  mesh containing 176 irreducible \(k\)  points to achieve energy convergence of 1 meV. As shown in Fig. 6(a-d), the calculated band structure and atom-resolved density of states (DOS) indicates that the system is a nonmagnetic semiconductor. There is a band gap of about 1 eV for the calculations without SOC [Fig. 6(a,b)] and about 0.9 eV for the case with SOC [Fig. 6(c,d)]. The total DOS (black line) rises rapidly from the band edges with the band character being mainly of Ir \(5d\)  orbitals. However, the contributions from Sb and Se \(p\)  orbitals are significant in reducing the Ir \(5d\)  orbital weight, compared with the dominant character of the transition metal in CoSbS and FeSbS [5]}. This weakens magnetic instability, if any, upon charge doping. The effects of SOC is more clearly seen in the band structures [Fig. 6(a,b)], where the SOC induces the band splitting is about 0.2 eV for the valence bands around the Fermi level. The band-structure plots also suggest that the valence and conduction bands are substantially massive. These are similar to the results for IrBiSe [6]}, where the SOC splitting of about 0.3 eV was reported. Thus, the split band in stoichiometric IrSbSe is also expected to be fully spin-polarized with 3D chiral spin texture [6]}. The present off-stoichiometric polycrystalline sample with a much smaller band gap suggests the occurrence of in-gap states.
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